Y. Minonishi

Plastic deformation of single crystals of Ti3Al with D019 structure

Abstract Single crystals have been successfully grown by floating-zone melting of an intermetallic compound Ti3Al which has the β- to α-phase transformation at about 1453 K. Single-crystal specimens of three major orientations were compressed in a temperature range from 300 to 1273 K and examined for the deformation characteristics of the three slip modes, namely {1100}〈1 120〉 prism slip, (0001)〈1 120〉 basal slip and {1122}〈1 123〉 pyramidal slip. Deformation morphology and dislocation arrangements were studied by optical microscopy and transmission electron microscopy respectively. The deformation behaviour and dislocation arrangements, which are very different from those reported on polycrystalline materials, are presented and discussed.

Elastic constants and their temperature dependence for the intermetallic compound Ti3Al

Abstract Single-crystal elastic contents of Ti3Al have been measured from 3.3 to 290K by a rectangular parallelepiped resonance method. The elastic anisotropy of Ti3Al is similar to, but slightly larger than, that of pure Ti. The values of the measured elastic constants are compared with those calculated for pure Ti stressed to such an extent that it has the same lattice constants as Ti3Al. The difference is interpreted as indicating the existence of directional bonding in the compound.

The core structure of 1/3⟨1123⟩ {1122} edge dislocations in h.c.p. metals

Abstract A computer simulation has been carried out for the core structure of a 1/3⟨1123⟩ {1122) edge dislocation using the truncated Lennard-Jones (12–6) potential. The core is composed of two distinct structural units which can be interpreted in terms of {1121) and {1122) twins. This core structure differs markedly from that proposed by Rosenbaum (1964).

Anomalous temperature dependence of the yield stress of Ti3Al by {1121} 〈1 126〉 slip

Abstract Ti3Al single crystals have been compressed along the c axis at temperatures from 300 to 1273 K. The yield stress shows an anomalous temperature dependence with a peak at around 1120 K. Slip planes are determined to be {1121} by two-surface trace analysis and dislocations observed have Burgers vector ⅓,〈1 126〉 and are mostly of edge character. A possible mechanism for the anomaly is briefly discussed.

The core structures of a ⅓〈1 123〉{1122} edge dislocation under applied shear stresses in an h.c.p. model crystal

Abstract The behaviour of a ⅓〈1 123〉{1122} edge dislocation under the effect of an external shear stress applied on {1122} planes has been studied by computer calculation. Before the application of the external stress, the dislocation has either of two typos of core configuration; one spreads as two twin faults on {1121} and {1122} planes (type I) and the other as a stacking fault with a fault vector of b/2, hounding partial cores still having the same twin faults as in the type-I core (type II). When the shear stress is applied in a direction that corresponds to tension along the c axis, the {1121) twin develops from both types of core under suitable conditions. When the stress is applied in the opposite direction, the type-I core changes into type-II; the dislocation moves as an extended dislocation without development of the {1122} twin. The results are interpreted and discussed in the light of experimental observations on deformation in h.c.p. metals.

〈1 123〉{1011} slip in zirconium

Abstract Electron microscopy observations were made on Zr polycrystals deformed at room temperature. Deformation substructure consists of ⅓〈1120〉 dislocations mostly in screw orientations, ⅓〈1123〉 dislocations on {1011} planes, and deformation twins with a small volume fraction. The observed configurations of the ⅓〈1123〉 dislocations indicate that their mobility is low when they lie along a close-packed direction, supported by a computer simulation study.

The core structure of a 1/3〈1 123〉 screw dislocation in h.c.p. metals

Abstract Atomistic calculations are made for a 1/3〈1 123〉 screw dislocation, using a truncated Lennard-Jones (12–6) potential. A variety of core configurations are formed depending on initial conditions. They can be interpreted as combinations of two types of elementary structures; one corresponds to an extension of the core along {1011}, and the other along 〈1122〉 planes, each bounded by partial dislocations with the Burgers vectors b/2 (= 1/6〈1 123〉). The fault energies on the {1011} and {1122} planes are almost the same, which is consistent with the appearance of various core configurations. The planar fault bounded by partials is not a simple stacking fault but has transition layers, which are caused by shuffling movements of atoms normal to the fault vector. The shear displacement accompanying the fault, however, occurs stepwise at the fault and the partials are not zonal. This is in sharp contrast to the dissociation models for the dislocation so far proposed.

Atomistic study of ⅓〈1 123〉{1011} dislocations in h.c.p. crystals. I. Structure of the dislocation cores

Abstract The core structure of ⅓〈1 123〉 dislocations on {1011} planes in h.c.p. crystals has been studied by computer simulation using a truncated Lennard-Jones potential. An edge dislocation and a mixed dislocation parallel to the [2113] direction (30°dislocation) extend over the (1011) plane. A mixed dislocation parallel to the [1210] direction (105° dislocation) assumes either of the following three configurations, depending on initial elastic dislocations introduced: (i) a planar configuration spread over the (1011) plane, (ii) a nonplanar configuration spread over the (1011), (0001) and (1010) planes, (iii) another nonplanar configuration in which the disregistry is not confined to any low-index planes. The result is interpreted in terms of stacking faults in h.c.p. crystals.

Fourfold dissociation of superlattice dislocations in Ti3Al

Abstract Weak-beam observation electron microscopy has been performed on superlattice dislocations lying on the prism planes in Ti3 Al single crystals deformed at room temperature. The dislocations observed are mostly of edge character. They are found to dissociate into four partials on the prism plane; a superdislocation into two unit dislocations with an associated antiphase boundary in between and the latter again into partials bounding a complex stacking fault.

An in-situ transmission electron microscopy study of pyramidal slip in Ti3Al: II. Fine structure of dislocations and dislocation loops

Abstract An in-situ transmission electron microscopy investigation has been conducted of the dynamic behaviour of dislocations in the type I pyramidal (π1) slip plane of Ti3Al single crystals which operates when specimens are strained in tension along the c-axis. Superpartial dislocations are split asymmetrically into two unlike partials. The high-energy complex stacking fault involved is thought to relax by short-range diffusion and this can account for the very strong disordering, the glide softening, and the nucleation of loops reported in part I. Three loop families have been identified, all vacancy type. Mechanisms are proposed to explain the formation of loops, their interactions with mobile dislocations, and their alignment along a c + a/2 direction which is different from the Burgers vector direction of the gliding dislocations. It is concluded that the critical resolved shear stress of pyramidal slip is determined by the nucleation process of loops.